1. Field of the Invention
This invention relates to prosthetic implants having a fixation stem, and in particular relates to a prosthetic implant having a fixation stem with decreased or varying stiffness such that the problems associated with stress shielding are reduced.
2. Description of the Related Art
For many years now, prostheses have been implanted in the human body to repair or reconstruct all or part of an articulating skeletal joint, such as the hip joint. The hip joint includes the femur and the pelvis, each of which has a surface for articulation against an adjacent articulation surface of the other bone. The femur has a head having a convex, generally spherically contoured articulation surface. The pelvis includes an acetabulum having a concave, generally spherically contoured articulation surface. The articulation surfaces of the femur and the pelvis form a ball-and-socket type joint.
One or both of the articulation surfaces of the hip joint may fail to perform properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface. In an artificial hip joint, a femoral implant can be used to replace the natural head and articulating surface of the femur, and an acetabular cup can be used to replace the natural socket and articulating surface of the acetabulum of the pelvis. The natural or artificial femoral head articulates directly against the natural acetabulum or the artificial acetabular cup.
A femoral implant may be affixed to the femur using bone cements. While bone cements provide the initial fixation necessary for healing following surgery, bone cements often result in a very stiff overall structure, are prone to loosening with time, and can provoke tissue reactions. Because of the disadvantages associated with the use of bone cements, “cementless” or “press fit” femoral implants have been developed. One well known “cementless” or “press fit” femoral implant includes a stem. The femoral implant stem “press-fits” into the intramedullary canal of the femur to hold the femoral implant rigidly in the femur.
A recognized problem with the use of an interference fit (press-fit) femoral implant stem is that transfer of stress from the femoral implant to the femur is abnormal. Instead of a normal loading of the femur primarily at the end of the femur near the joint surface, the femur is loaded more distally where the stem of the implant contacts the femur. This results in a phenomenon called “stress shielding” in which the load (i.e., stress) bypasses or “unloads” the end of the joint surface portion of the femur. As a result, the joint surface portion of the femur undergoes resorption, (i.e., the femur retreats from its tight fit around the implant stem) thereby introducing some “play” into the fit. This leads to weakening over a period of years, thus creating a potential for fracture or a loosening of the femoral implant within the femur.
It has been reported that one cause of stress shielding is the high bending stiffness of conventional femoral implant stems. In particular, the relatively high mechanical stiffness of the stem portion of metallic femoral implants tends to stress protect (i.e., “unload”) the proximal femur which can lead to resorption and loosening of the femoral implant. Because stem stiffness increases exponentially in relation to stem diameter, patients with relatively large intramedullary canals, which require a femoral implant with a large diameter stem for optimal fit, can be particularly susceptible to this phenomenon of “stress shielding”. In extreme cases, the proximal femoral bone may resorb to a small fraction of its original mass, possibly causing a loss of support of the implant or implant breakage.
In order to eliminate the stress shielding problems associated with the high mechanical stiffness of the stem portion of metallic femoral implants, it has been proposed to produce a femoral implant having a stem with reduced mechanical stiffness or varying mechanical stiffness along the length of the stem. For example, U.S. Pat. Nos. 5,702,482, 5,509,935, 5,336,265, 5,007,931, 4,921,501 and 4,808,186 disclose femoral implants including a stem with longitudinal slots or channels that serve to decrease the stiffness of the stem. The reduced stiffness of the stem decreases stress shielding. It has also been proposed in U.S. Pat. Nos. 5,725,586, 5,316,550 and 5,092,899 that the stiffness of the stem of a femoral implant can be reduced by hollowing out the interior of the stem.
While each of these patents may provide a solution to the stress shielding problems associated with the high mechanical stiffness of the stem of a femoral implant, these stemmed implants do have certain disadvantages. For instance, implants that include a stem with longitudinal slots may not provide for a snug press fit and may not provide sufficient surface area for bone ingrowth. Also, hollowing out the stem of a femoral implant may only be marginally effective at reducing stress shielding, due to the fact that the centrally located material contributes little to the stiffness of the implant.
Therefore, there is a continuing need for an improved prosthetic implant having a fixation stem with reduced stiffness such that the stress shielding problems associated with the high mechanical stiffness of the stem can be minimized. In particular, there is a need for an improved prosthetic implant having a fixation stem wherein the stiffness at the distal end of the stem is reduced such that the proximal end of the stem may bear more load.